A wide variety of flexible electrical circuits (also sometimes known as flex circuits) are well known and used in many different applications. The structure of a relatively simple flexible circuit has one or more electrical leads or traces (e.g., formed of copper or other conductive metal) covered on one or both sides by a protective coverlay (also sometimes referred to a coverlayer or a cover coat). The coverlay is typically a dielectric material such as polyimide. A more complex flexible circuit structure of this type has a dielectric base layer on one side of the electrical leads or traces, and a coverlay on the other side. Still other known flexible circuits have a dielectric base layer without a coverlay.
Integrated lead or wireless suspensions used to support read/write transducers in disk drives or other dynamic data storage systems are another type of flexible circuit. These devices typically have a stainless steel or other spring metal layer for supporting the electrical traces. A layer of dielectric insulating material separates the traces from the stainless steel. A protective dielectric coverlay is commonly applied over the traces.
Unfortunately, due to the differences in the thermal and hygroscopic expansion properties of the different layers of material in a flexible circuit, the circuits are susceptible to curl. For example, when the coverlay material is cured during its application to the flexible circuit, or when the coverlay is cooled or dried after application to the flexible circuit, the length of the coverlay can decrease more than the conductor layer, and cause the flexible circuit to curl toward the coverlay. Conversely, if the coverlay is heated or hydrated, its length can increase more than the conductor layer, and cause the flexible circuit to curl away from the coverlay. This curling can interfere with mechanical or other characteristics of the flexible circuit or other structures to which the flexible circuit is mounted. In the case of integrated lead suspensions in particular, this curl can cause changes in important operational parameters such as the static attitude and gram load of the suspension. There is, therefore, a need for integrated lead suspensions and other flexible circuits having reduced susceptibility to curl. To be commercially viable, any such flexible circuit should be efficient to manufacture.